Photochemical reactions are attractive for drug discovery because they enable the synthesis of unusual, strained molecular frameworks that cannot be synthesized by other methods. Nevertheless, pharmaceutical companies rarely take advantage of photochemical synthesis because the need for specialized photochemical equipment is an impractical impediment and because very few methods to control the stereochemistry of photochemical reactions exist. Therefore, the structures of the molecules that are produced by photochemical synthesis have essentially not been examined as possible drug candidates. Our laboratory has pioneered a new approach to photochemical synthesis that uses convenient, readily available sources of visible light (e.g., household lightbulbs, ambient sunlight) in order to make photochemical synthesis accessible to medicinal chemists for the first time. This Proposal has two parallel objectives: Aim 1. The development of strategies to control the stereochemistry of photochemical reactions. Aim 2. The development of a novel mechanism of photochemical activation with visible light. These methods are powerful, robust, and simple to perform on large, industrially relevant scales. Thus, the research described in this proposal will significantly impact both the academic chemistry community and the broader community of medicinal chemists who require new methods of molecule construction to discover the next generation of life-saving drugs.